1. Field of the Preferred Embodiment(s)
This invention relates to oscillators which provide a stable reference frequency signal in computers or other electronic equipment. Specifically, there is an ovenized oscillator that has two ovens. One of the ovens is heated and the other is cooled by a thermoelectric module.
2. Description of the Related Art
Various devices are well known for providing a reference frequency or source such devices are called oscillators. The oscillator typically has a quartz crystal or other resonator and also has electronic compensation circuitry to stabilize the output frequency.
Various methods are known to stabilize the output frequency as the temperature of the oscillator changes. Temperature compensated crystal oscillators (TCXO""s) typically employ a thermistor network to generate a correction voltage which reduces the frequency variation over temperature. The correction voltage is usually applied to a varactor diode in the crystal circuit such that the crystal frequency may be varied by a small amount. TCXO stability can approach 0.1 PPM but several problems must be addressed. A TCXO that resides at one temperature extreme for an extended period of time may exhibit a frequency shift when returned to normal room temperature. Usually this hysterisis or xe2x80x9cretracexe2x80x9d error is temporary but a seemingly permanent offset is common. Retrace errors are usually less than about 0.1 PPM but can be much higher, especially if the mechanical tuning device (trimmer capacitor or potentiometer) is shifting. This hysterisis makes the manufacture of TCXOs with specifications approaching 0.1 PPM quite difficult. The high precision crystals found in oven oscillators exhibit less retrace but they are unsuitable for use in TCXOs because they often exhibit activity dips at temperatures below the designed oven temperature and SC-cuts and high overtone types cannot be tuned by a sufficient amount to compensate for the frequency excursion. In addition SC cut crystals are very expensive. TCXOs are preferred to oven oscillators in low power applications and when a warm-up period is not acceptable. The only warm-up time is the time required for the components to reach thermal equilibrium and the total current consumption can be very lowxe2x80x94often determined by the output signal power requirements. Older TCXO designs employ from one to three thermistors to flatten the crystal temperature frequency curve. Newer designs employ digital logic or a microprocessor to derive a correction voltage from values or coefficients stored in memory.
Ovenized oscillators heat the temperature sensitive portions of the oscillator which is isolated from the ambient to a uniform temperature to obtain a more stable output. Ovenized oscillators contain a heater, a temperature sensor and circuitry to control the heater. The temperature control circuitry holds the crystal and critical circuitry at a precise, constant temperature. The best controllers are proportional, providing a steady heating current which changes with the ambient temperature to hold the oven at a precise set-point, usually about 10 degrees above the highest expected ambient temperature. Temperature induced frequency variations can be greatly reduced by an amount approaching the thermal gain of the oven. The crystal for the oven is selected to have a xe2x80x9cturning-pointxe2x80x9d at or near the oven temperature further reducing the sensitivity to temperature. The combination of the high oven gain with operation near turning point yields temperature stabilities as good as 0.0001 PPM over a temperature range that would cause the crystal to change by 10 PPM. Highly polished, high-Q crystals which often have significant activity dips may be designed with no activity dips near the operating temperature providing better stability and phase noise than crystals designed for wide temperature ranges. Ovenized oscillators allow the use of SC-cut crystals which offer superior characteristics but which are impractical for ordinary TCXOs because of their steep frequency drop at cooler temperatures. Unfortunately SC cut crystals are much more expensive to produce than AT cut crystals typically used in TCXO""s. Oven oscillators have a higher power consumption than temperature compensated oscillators. Oven oscillators require a few minutes to warm-up and the power consumption is typically one or two watts at room temperature.
In order to improve frequency stabilities beyond that of an regular ovenized oscillator, ovenized oscillators using 2 ovens have been designed. In a double oven configuration, 2 insulated enclosures are placed inside each other with a proportionally controlled heater assembly for each. In order to maintain temperature control of the assembly under varying ambient conditions, it is necessary to maintain a differential of about 10 degrees Celsius between the highest ambient temperature to be experienced and the setpoint of the outer oven. Another 10 degrees Celsius differential is then required between the outer oven and the inner oven. The total heat rise above ambient between the crystal and the outside of the outer oven is more than 20 degrees Celsius. If the ambient temperature is high, the inner oven may need to operate at temperatures of around 110 degrees Celsius. There are several significant disadvantages of operating an oscillator at this high of a temperature. First, the reliability of electronic components decreases with temperature. The mean time between failures (MTBF) or operating lifetime of the circuit assembly is reduced as the temperature increases. In order to improve the failure rate, higher grade components must be used or more time spent screening components. Second, aging of the crystal resonator is accelerated. Crystals age more rapidly at higher temperatures. As the crystal ages, its frequency shifts causing frequency stability to rapidly degrade.
Despite the advantages of the prior art oscillators, an unmet need exists for a dual oven oscillator that can operate with high frequency stability at high ambient temperatures.
It is a feature of the invention to provide an ovenized oscillator that has two ovens. One of the ovens is heated and the other is cooled by a thermoelectric module.
A further feature of the invention is to provide an ovenized oscillator assembly that includes an oscillator for producing a reference frequency. A first temperature controlled oven has an oscillator located inside. A thermoelectric heat pump is in thermal communication with the first temperature controlled oven. The thermoelectric heat pump keeps the first oven at a first temperature. The thermoelectric heat pump and first oven are located inside the second temperature controlled oven. A heater is in thermal communication with the second temperature controlled oven. The heater keeps the second oven at a second temperature. The first temperature is less than the second temperature.
A further feature of the invention is to provide an ovenized oscillator assembly that includes an outer temperature controlled oven and an inner temperature controlled oven contained within the outer temperature controlled oven. An oscillator is located inside the inner temperature controlled oven. The temperature of the outer oven is higher than the temperature of the inner oven.
A further feature of the invention is to provide a method of operating an oscillator to provide a reference frequency signal that includes: providing an oscillator operable to produce a reference frequency and a first and a second temperature controlled oven. The oscillator is located inside the first temperature controlled oven. The first oven is located inside the second oven. Cooling the first oven and heating the second oven such that a first oven temperature is lower than a second oven temperature.
The invention resides not in any one of these features per se, but rather in the particular combination of all of them herein disclosed and claimed. Those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. Further, the abstract is neither intended to define the invention of the application, which is measured by the claims, neither is it intended to be limiting as to the scope of the invention in any way.